BGA rework: how to determine the right temperature profile

During BGA rework, you need to apply enough heat to reliably melt all solder joints underneath the component. At the same time, you want to limit the thermal stress on the PCB, BGA and surrounding components.

This makes the temperature profile one of the most important parameters in the rework process.

However, there is no universal temperature profile. The correct settings depend on the PCB, component, solder alloy and the thermal conditions around the BGA.

The key principle is therefore: do not start from a fixed temperature setting, but from the actual temperature profile measured on the assembly.

Why can’t you simply use the original reflow profile?

During the original assembly process, the entire PCB passes through a controlled reflow process. BGA rework creates a different thermal situation.

The PCB is already fully assembled. Components surrounding the BGA may be sensitive to another thermal cycle. In addition, heating is applied locally, while the rest of the PCB absorbs and dissipates part of that heat.

A profile that worked during the original reflow process is therefore not automatically suitable for BGA rework.

The profiles for desoldering and soldering the replacement BGA do not necessarily have to be identical either. During desoldering, the existing joints need to become sufficiently liquid to remove the component safely. When placing a new BGA, new solder joints need to be formed.

Start with the thermal mass of the PCB

One of the first factors to assess is thermal mass.

A thick multilayer PCB with a large amount of copper absorbs more energy and distributes heat differently from a thin PCB with less copper.

This directly influences how quickly the assembly can be heated.

For a PCB with a high thermal mass, a lower heating gradient may be necessary. This gives the heat sufficient time to spread through the assembly and helps limit large temperature differences.

A thin PCB with a low thermal mass responds more quickly and therefore requires different settings.

Before creating a profile, consider factors such as:

• PCB thickness and construction;
• amount of copper;
• size of the BGA;
• surrounding components;
• materials used;
• possible moisture absorption.

Moisture deserves particular attention. Heating too quickly can create internal pressure and mechanical stress, potentially damaging the component or PCB.

Measure the temperature on the PCB, not just at the heater

The set temperature of the heating element does not tell you what temperature the BGA actually reaches.

A thermocouple should therefore be positioned as close as possible to the component. This allows you to monitor how the temperature on the assembly actually develops during the process.

This distinction is important.

The heater itself can be considerably hotter than the measured temperature near the BGA because part of the energy is absorbed by the PCB and surrounding components.

A controlled rework process therefore does not rely solely on heater settings, but on the measured temperature profile on the PCB.

Why use both top and bottom heating?

BGA rework typically uses both top and bottom heating.

Bottom heating helps bring the PCB gradually up to temperature. The top heater can then apply energy more specifically to the BGA area.

Particularly with a PCB that has a high thermal mass, this prevents all the required energy from having to be introduced locally from above.

The aim is controlled heat distribution: sufficient energy at the solder joints without unnecessarily exposing the BGA or surrounding components to excessive temperatures.

Temperature-sensitive components nearby can be shielded or cooled where necessary.

How quickly can you heat a BGA?

A gradual, almost linear temperature profile is often a useful starting point for BGA rework.

For an assembly with a higher thermal mass, a heating rate of around 1°C per second or lower can be used as a starting point. If more time is needed to distribute the heat through the PCB, the gradient can be reduced further, for example towards 0.7°C/s.

These are not universal settings.

The appropriate gradient depends on the actual PCB and component. Materials also expand at different rates during heating. Large temperature differences can therefore introduce mechanical stress.

Heating as quickly as possible is not the objective. Controlled and sufficiently uniform heating is more important.

How long should the solder remain above liquidus?

Liquidus is the temperature above which the solder alloy is fully liquid.

In conventional reflow processes, longer times above liquidus may be used. During BGA rework, the required time can be shorter because solder is already present in the joints.

As a practical indication, a time above liquidus of approximately 20 to 40 seconds can be considered for BGA rework.

Again, this should not be treated as a fixed setting. The required time depends on the solder alloy, thermal mass and purpose of the profile.

During desoldering, for example, the joints only need to remain liquid long enough to allow the BGA to be removed safely.

Which peak temperature should you use?

The peak temperature depends on the solder alloy and the temperature limits of the PCB and components.

You need to exceed the melting point sufficiently to achieve complete reflow, but heating beyond what is necessary increases thermal stress.

For SAC305, for example, the melting temperature is around 219°C. A possible peak temperature can be around 239°C, while it is generally advisable to remain below approximately 245°C where possible.

These values should be treated as process guidelines, not as a universal BGA profile.

The component datasheet, solder alloy and measured temperature on the actual assembly remain decisive.

A good BGA profile is measured, not copied

One of the main mistakes when creating a BGA rework profile is looking for a single set of settings that can be applied to every PCB.

Such a profile does not exist.

A suitable profile is created by assessing the assembly, selecting an initial profile, measuring the actual temperature close to the BGA and then adjusting the process accordingly.

The key parameters need to be considered together:

thermal mass → heating rate → liquidus → peak temperature → thermal load

Smans can support you in creating, measuring and optimising temperature profiles for BGA rework. This allows the profile to be adapted to the actual PCB and component being reworked rather than relying on a generic setting.